Safest bet?
There's no such thing but using up your own common sense then Bitcoin would be the safest choice but of course it wont really guarantee out that you would be making money in 2 years but
seeing potential of bitcoin for a long time then its high chance that you could make money but dont expect about 4x average on short years basis.
Its up to you if you would be touching up other altcoins for you to chase up that kind of multiplier because if you do stick into those top ranking or established
coins then you cant really see those things on short term basis.
there's no safest bet here especially the crypto market. better diversify your portfolio, and invest outside the crypto market. 2 years is a long time in crypto, but if you happen to invest some in real-estate, this will be a good one as real estate properties tend to increase its value over time.
for cryptocurrencies, diversify it and if he wants, he can put more on btc. eth and sol are good ones too. it is on him how much he will allot to these projects.